CHAPTER XXIV
THE UPTILT OF THE LAND AT THE CLOSE OF THE ICE AGE
The response of the earth’s shell to its ice mantle.—There is now good reason to believe that the earth’s outer shell makes a response by oscillations of level due to the loading by ice, on the one hand, and to the removal of this burden upon the other. We know, at least, that both in northern Europe and in North America areas which have undergone depression during and elevation after the ice age, correspond closely to the regions which were ice covered. Wherever in these regions there was high relief before the advent of the ice, river valleys were drowned at the land margins and were also gouged out into troughs through erosion by the outlet tongues upon the margin of the ice sheet. Such furrowed and half-submerged valleys have a characteristic U-shaped section, so that their walls rise precipitously from the sea. From their typical occurrence in Scandinavian countries the name fjord has been applied to them.
It is now no less clear that the removal of the ice blanket brought from the earth a relatively quick response in uplift, which began before the ice front had retired across the present international boundary of the United States, and that this uplift continued until the final disappearance of the ice. A far slower elevation of a somewhat different nature has continued, even to the present day.
It is obvious that at the time of their formation all shore lines referable to the work of waves must have been horizontal, and hence any variations from a perfect level which they reveal to-day must indicate that a tilting movement of the ground has occurred since the waters departed from their basins. We have thus provided for us in the positions of these ancient water planes, particularly because of their wide extent, a complete record the refinement of which is not easily overstated. Interpreting this record, we find that it was the uptilt of the land to the northward which brought the glacial lake history to an end and inaugurated the present system of St. Lawrence drainage. The outlet of the Nipissing Great Lakes is to-day more than a hundred feet above the level of the outlet at Port Huron, where the upper lakes are now discharging their waters, and this difference in level can only be ascribed to an upward tilting of the land since the latest of the glacial lake stages.
The abandoned strands as they appear to-day.—The traveler by steamer upon the upper lakes, as he comes within view of each rocky headland, may note how the profile against the horizon is notched by a series of steps or terraces ([Fig. 368]), and if he has followed the discussion in previous chapters, he will suspect that these terraces mark the now abandoned shore lines which have come to their present position through a series of uplifts of the ground accompanied by earthquake shocks. As his steamer skirts the shore he may chance to note a cave within the rock cliff which represents the now elevated sea-arch of an ancient shore.
Fig. 368.—The notched rock headland of Boyer Bluff between Green Bay and Lake Michigan (after Goldthwait).